Contact lens and methods of manufacture and fitting such lenses and computer program product

ABSTRACT

The invention is directed to a contact lens design and methods of manufacturing, fitting and using such lenses. The contact lens may be designed for use in a corneal refractive therapy (CRT) program. The lens provides a design which allows proper fitting of a patient, whether for corrective contact lenses or for a CRT program. Due to the rational design of the lenses according to the present invention, a minimal number of lens parameter increments can be identified to cover the range of common corneas. It is therefore possible to provide pre-formed lens buttons or blanks which are easily formed into a final design, thereby simplifying and speeding up the treatment process. Further, any adjustment of the lens design which may be required based upon trial fitting or the like, is easily envisioned and implemented by the fitter.

CROSS REFERENCE

This application is a divisional of co-pending U.S. application Ser. No.12/560,039 filed Sep. 15, 2009, which is a divisional of co-pending U.S.application Ser. No. 11/836,866 filed Aug. 10, 2007, which is acontinuation of U.S. application Ser. No. 11/347,955 filed Feb. 6, 2006,now issued as U.S. Pat. No. 7,270,412 which issued Sep. 18, 2007, whichis a divisional of U.S. application Ser. No. 09/894,351 filed Jun. 27,2001, now U.S. Pat. No. 7,040,755 which issued May 9, 2006, which inturn claims the benefit of U.S. Provisional Application Ser. No.60/214,554, filed Jun. 27, 2000. Additionally, co-Pending U.S.application Ser. No. 11/856,510 filed Sep. 17, 2007 is a continuation ofU.S. application Ser. No. 11/347,955 filed Feb. 6, 2006, all of whichare hereby incorporated herein by reference.

TECHNICAL FIELD

This invention relates to contact lenses and methods of manufacture, aswell as methods for reshaping the cornea of an eye to treat visualacuity deficiencies. The invention is more particularly related tonon-surgical methods of reshaping the cornea. This procedure maybereferred to as Corneal Refractive Therapy (CRT) when the therapy relatesto designing and fitting a single contact lens to reshape the cornea,and/or orthokeratology (ortho K) when referring to the use of a seriesof lenses for the purpose of reshaping the cornea. The invention furtherrelates to methods of fitting contact lenses and designing such lenses,as well as a software product for designing such lenses.

BACKGROUND OF THE INVENTION

In the treatment of visual acuity deficiencies, correction by means ofeyeglasses or contact lenses are used by a large percentage of thepopulation, Such deficiencies include patients having hyperopia or beingfar-sighted, myopia or near-sighted patients as well as astigmatismscaused by asymmetry of the patient's eye and presbyopia caused by lossof accommodation by the crystalline lens. Although the use of contactlenses is widespread, there are potential difficulties in properlyfitting a lens for a patient, which in turn could damage the patient'scornea or cause discomfort. More recently, to alleviate the burden ofwearing eyeglasses and/or contact lenses, surgical techniques have beendeveloped for altering the shape of the patient's cornea in an attemptto correct refractive errors of the eye. Such surgical techniquesinclude photorefractive keratectomy (PRK), LASIK (laser in-situkeratectomy), as well as procedures such as automated lamilarkeratectomy (ALK) or implanted corneal rings, implanted contact lenses,and radial keratotomy. These procedures are intended to surgicallymodify the curvature of the cornea to reduce or eliminate visualdefects. The popularity of such techniques has increased greatly, butstill carry risk in both the procedure itself as well as post surgicalcomplications.

Alternatives to permanent surgical procedures to alter the shape of thecornea include CRT and ortho-K, where a contact lens is applied to theeye to alter the shape or curvature of the cornea by compression of thecorneal surface imparted by the lens. The reshaping of the cornea inorthokeratology has been practiced for many years, but typically hasrequired a series of lenses and an extensive period of time to reshapethe cornea. It is also typical of orthokeratology treatment plans thatthe lenses used for reshaping of the cornea must be custom designed andmanufactured, thereby greatly increasing the cost and complicatinggeneral use of such procedures. Further, orthokeratology lensestypically have various deficiencies, particularly relating to properlydesigning a lens for a particular patient to achieve best results in thetreatment process. Specifically corneal abrasions from poorlydistributed bearing, corneal warpage from decentered lenses, edema fromtight fitting lenses and discomfort from excessive lens edge standoffare problems associated with an improperly fit lens. The design oforthokeratology lenses have not lent themselves to be easily fitted fora particular patient and their needs, requiring a doctor or otherpractitioner to have significant skill in complex geometric computationto properly mate the lens shape to the patients cornea and a high levelof expertise in properly fitting a patient. Further, even with a highlevel of expertise, a lens designer many times will design a lens whichwill not work properly with a patient, and must be redesigned to accountfor the errors of the original design. Such a process is lengthy andincreases the cost of the treatment correspondingly. It would bedesirable to provide a lens for corneal refractive therapy which wouldallow a novice fitter to more easily select and arrive at a final designto simplify the fitting process.

Another deficiency of Ortho-K lenses is found in the complexity of thedesigns, which exacerbate the fitting problems mentioned previously. Inthe fitting process, if there is an aspect of the lens design which isnot properly fitted for the desired treatment of the patients eye, orcauses excessive discomfort to the patient, the lens must be redesignedaccordingly. Unfortunately, in an attempt to redesign a lens, apractitioner may affect other aspects of the lens due to theinterdependency between design features or may not anticipatemanufacturing variances required by the altered design. It would beworthwhile to provide an CRT lens having independent features, whichcould be easily modified if required to attain a proper fit in a simplerand more cost-effective process. It would also be desirable to providean CRT lens design which enhances the ability of a fitter and aconsultant to discuss more clearly the lens cornea relationship, toenable parameters of the lens to be easily communicated to a finishinglaboratory for forming a desired lens.

SUMMARY OF THE INVENTION

In accordance with the foregoing, it is an object of the presentinvention to provide a CRT contact lens, method of manufacturing suchlenses and methods of designing and fitting lenses and a softwareproduct for design of such a lens. The contact lens according to theinvention overcomes the deficiencies of the prior art, and provides adesign which allows proper fitting of a patient, whether for correctivecontact lenses or for use in an orthokeratology treatment program. Theability to properly fit a patient will alleviate, at least to a greatdegree, corneal abrasions from poorly distributed bearing, cornealwarpage from decentered lenses, edema from tight fitting lenses anddiscomfort from excessive lens edge standoff. The simplified designallows a novice or relatively unskilled fitter to visualize therelationship between the contact lens and cornea of a patient's eye. Thedesign and corresponding relationship to the patient's cornea allowsselection of original trial lenses and any subsequent modifications tobe easily designed or corrected. The lens design also provides improvedability of a fitter to consult with a lens designer to discuss clearlythe lens cornea relationship for determining of the lens design. Due tothe rational design of the lenses according to the present invention, aminimal number of lens parameter increments can be identified to coverthe range of common corneas. It is therefore possible to providepre-formed lens buttons or blanks which are easily formed into a finaldesign, thereby simplifying and speeding up the treatment process.Further, any adjustment of the lens design which may be required basedupon trial fitting or the like, is easily envisioned and communicated bythe fitter and fabricated by the manufacturer.

In accordance with this and other objects of the invention, there isprovided a contact lens comprising 1) a central zone having a posteriorsurface having a curvature determined by the correction or reshaping tobe imparted to the cornea; 2) a connecting zone is provided adjacent andconcentric to the central zone, the connecting zone having a shapedefined by a first generally posteriorly concave portion adjacent thecentral zone (this concave portion being initially of longer radius thanthe central zone then becoming steeper than the central zone untilnearly parallel to the central axis of the lens) and transitioning to agenerally posteriorly convex portion thus having the appearance of anelongated backward “S” or sigmoidal shaped curve; and 3) a peripheralzone is provided adjacent and concentric to the connecting zone, and isprovided with a conoid shape. In a CRT lens, the peripheral zone is usedto facilitate redistribution of the corneal tissue by the central zoneand to minimize the potential for its extreme edge or its junction withthe first annular zone to impinge directly on the cornea even aftercontact.

In another aspect, the invention provides a contact lens having acentral zone and first and second annular zones, wherein the secondannular zone is initially positioned so as not to engage the cornea, andshaped such that only after the majority of redistribution of cornealtissue by the central zone is accomplished will the second annular zonecontact the cornea acting to neutralize forces imparted on the cornea bythe central zone. The first annular zone connects the central zone andthe second annular zone in the contact lens.

The invention further relates to, a method for altering the shape of apatient's cornea comprising the steps of determining the present shapeof the cornea and a desired corrected shape therefore. Thereafter,imparting a force to the cornea to alter its shape by means of a contactlens comprising a central zone having a curvature corresponding to thedesired corrected shape, and first and second annular zones concentricthereto. The second annular zone is positioned relative to the corneaand shaped such that upon redistribution of corneal tissue by thecentral zone, the second annular zone will contact the cornea acting toneutralize forces imparted thereon with minimal alteration of theperipheral cornea. The first annular zone connects the central zone tothe second annular zone in the lens.

The invention also relates to a method for treating visual acuitydeficiencies by wearing a contact lens for an amount of time to modifythe shape of the cornea in a predetermined manner. In this method, thesteps of providing a lens with a central zone having a shape designed toimpart force on the cornea and an annular peripheral zone positionedrelative to the central zone and shaped to selectively contact thecornea after an amount of redistribution of the corneal tissue by theforce applied thereto. An annular connecting zone connects the centralzone with the peripheral zone.

The invention also relates to a method for fitting a contact lensincluding enabling adjustment and visualization of the effect ofadjustments on the fit of the lens to a patient's eye. The methodenables a fitting technician to be easily taught as to the parameters ofthe lens which are modifiable to allow proper fitting based upon thecharacteristics of the patient's eye, and to assess and communicate apreferred geometry based upon the patient's characteristics. As anexample, the sagittal depth of the contact lens can be adjusted andassessed relative to the patient's eye for proper fitting by changingthe axial length of the connecting or first annular zone provided in thelens geometry, without altering the characteristics of the central andperipheral zones in the lens design and without altering the location ofengagement of the central and second peripheral zones that had beenobserved with the unadjusted lens. Alternatively, although an embodimentof the invention restricts changes to the widths of the connecting zoneand the central zone, in some cases it may be necessary to alter thevolume distribution under the connecting zone. In such cases the centraland connecting zone can be adjusted and assessed to allow proper tearflow and oxygen transmission beneath the lens by adjusting the diameterof the central zone, the axial length of the connecting zone and/or theradial width of the connecting zone. The fitting of the lens may also bedirected to adjusting the location of ultimate peripheral tangentialcontact by the conoid peripheral zone of the lens geometry with thecornea by adjusting the angle made by the peripheral zone to the centralaxis of the lens. The method of fitting is also provided by implementingchanges in a lens set having the central zone diameter, connecting zonewidth, lens diameter and edge profile provided with predeterminedshapes, measuring central corneal curvature of the patient's cornea,computing the preferred corneal curvature needed to eliminate refractiveerror for a patient and determining only two additional parameters,connecting zone depth and peripheral zone angle. These parameters areidentified by trial fitting lenses from a lens set having the centralzone diameter, connecting zone width, lens diameter and edge profileprovided with fixed dimensions. The needed parameters of connecting zonedepth and peripheral zone angle may be derived by fitting lenses fromsuch a fitting set having a fixed connecting zone depth with a series ofbased curves or alternatively, the fitting set may have a fixed basecurve and a series of connecting zone depths. In addition to one oranother of the two sets just described, another set having a fixed basecurve, a fixed connecting zone depth with a series of pheripheral zoneangles from which the final angle selection is derived may be provided.Alternatively for angle selection, one of or the other of these sets maybe configured to have a plurality of visible concentric rings,substantially allowing a determination of the lens diameter at whichsubstantially tangential touch occurs between the lens and the corneathereby making it possible to compute the correct angle of the at leastone peripheral zone. Alternatively, although an embodiment of theinvention restricts changes to the widths of the connecting zone and thecentral zone, in some cases it may be necessary to alter the volumedistribution under the connecting zone or the size of the treatmentzone. In such cases the central and connecting zone can be adjusted andassessed to allow proper tear flow and oxygen transmission beneath thelens by adjusting the diameter of the central zone, the axial length ofthe connecting zone and/or the radial width of the connecting zone.

As a further aspect of the invention, there is provided a computerprogram product and methods for designing and fitting a contact lens.The computer program product comprises a computer usable storage mediumhaving computer readable program code means embodied in the medium. Thecomputer readable program code means comprises code, responsive to userinputs, for modeling a contact lens to have a central zone with acurvature selected to impart force upon a patients cornea, and first andsecond annular zones. The second annular zone is positioned relative tothe central zone, and is modeled to have a shape to fit the patient'seye in a predetermined manner to provide centration or to selectivelyengage the cornea upon altering its shape a predetermined amount. Afirst annular zone is designed to connect the central zone to the secondannular zone. There is also provided computer readable program code forcalculating cutting parameters for a lathe used to produce the lens froma blank of material. There is also provided a computer readable programto use observations made with the fitting set lenses to compute theparameters most preferred for the patient. There is also provided acomputer readable program for employing data supplied by a topographerto compute the parameters most preferred for the patient.

This computer program may have the following inputs: 1) Flattestkeratometry reading; 2) manifest refractive sphere error (in minuscylinder form); 3) target final refractive error; and 4) Horizontalvisible iris diameter (HVID); the fifth input varies depending onwhether the fitting lenses used had variable based curves or variableconnecting zone depths. In the case of the former the input identifiesthe base curve of the lens observed to just give simultaneous apical andtangential touch, or in the case of the latter the input identifies theconnecting zone depth of the lens observed to just give simultaneousapical and tangential touch. The sixth input also depends on whichfitting lens set type is used, concentric rings or variable angles. Inthe case of the former the input is the diameter of tangential touch bythe lens having the concentric rings or in the case of the latter theinput identified the peripheral angle observed to meet the criteria oftouch diameter relative to lens diameter.

In either configuration the program output by computation is: 1) Lenspoer to order, 2) Base curve to order; 3) Connecting zone depth toorder; 4) peripheral zone angle to order; and 5) overall diameter toorder.

These aspects of the invention along with other objects and advantagesthereof will become apparent upon a further reading of the descriptionof the invention in conjunction with drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an embodiment of a contact lens according tothe invention.

FIG. 2 is a cross-sectional side view of the embodiment as shown in FIG.1.

FIG. 3 shows an enlarged side view of the central portion of the contactlens as shown in FIG. 1.

FIGS. 4A and 4B show a partial cross-section of a lens as part of a setof fitting lenses.

FIG. 5A shows a schematic illustration of the connecting zone in thelens of the invention.

FIG. 5B shows an enlarged side view of the annular connecting zoneaccording to the invention.

FIG. 5C is a schematic diagram showing the design of the connectingzone.

FIG. 6 is a diagram showing the conoid peripheral zone.

FIG. 7 is a diagrammatic illustration of an edge zone in the peripheralzone of a lens according to an embodiment of the invention.

FIG. 8A-8C show schematically the relationship between the peripheralzone and the corneal surface of a patient.

FIGS. 9-13 show the lens design for a first example of the presentinvention, and include a spreadsheet of lens parameters, a graph showingthe elevation of the lens zones from the cornea, a plot of theindividual and cumulative volumes under the lens zones, a semi-meridiancross section of the lens and a plot showing the front and back curvesin each of the zones of the lens.

FIGS. 14-18 show the lens design for a second example of the presentinvention, and include a spreadsheet of lens parameters, a graph showingthe elevation of the lens zones from the cornea, a plot of theindividual and cumulative volumes under the lens zones, a semi-meridiancross section of the lens and a plot showing the front and back curvesin each of the zones of the lens.

FIGS. 19-23 show the lens design for a third example of the presentinvention, and include a spreadsheet of lens parameters, a graph showingthe elevation of the lens zones from the cornea, a plot of theindividual and cumulative volumes under the lens zones, a semi-meridiancross section of the lens and a plot showing the front and back curvesin each of the zones of the lens.

FIGS. 24-28 show the lens design for a fourth example of the presentinvention, and include a spreadsheet of lens parameters, a graph showingthe elevation of the lens zones from the cornea, a plot of theindividual and cumulative volumes under the lens zones, a semi-meridiancross section of the lens and a plot showing the front and back curvesin each of the zones of the lens.

FIG. 29 is a partial cross section of the edge of the lens as shown inthe example of FIGS. 24-28.

FIGS. 30-34 show the lens design for a fifth example of the presentinvention, and include a spreadsheet of lens parameters, a graph showingthe elevation of the lens zones from the cornea, a plot of theindividual and cumulative volumes under the lens zones, a semi-meridiancross section of the lens and a plot showing the front and back curvesin each of the zones of the lens.

FIGS. 35-39 show the lens design for a sixth example of the presentinvention, and include a spreadsheet of lens parameters, a graph showingthe elevation of the lens zones from the cornea, a plot of theindividual and cumulative volumes under the lens zones, a semi-meridiancross section of the lens and a plot showing the front and back curvesin each of the zones of the lens.

FIG. 40 shows a method of fitting a patient in an embodiment of theinvention.

FIG. 41 shows a schematic representation of a patient's eye and the lensaccording to the invention for visualizing the fit therebetween.

FIG. 42 shows a flowchart for a computer program according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the invention, the contact lens anddesigning and fitting methods refer to a CRT lens design, but it shouldbe understood that the lens according to the invention could also bedesigned to simply provide vision correction in a manner similar totypical contact lenses, but designed according to the principles of theinvention and providing better centration, comfort or other advantages.Referring now to FIGS. 1 and 2, there is shown a first embodiment of acontact lens for positioning on a patient's cornea for reshaping thecornea to improve visual acuity. The lens 10 in general is dimensionedwithin normal ranges for corneal contact lenses, with an outsidediameter generally between 7 to 13 mm, and generally in the rangebetween 9.5 to 12 millimeters. More particularly, the diameter willnormally be chosen to be as large as possible, but no larger than thehorizontal visible iris diameter (usually 1 mm less) and to extendbeyond the point of ultimate tangential contact by the peripheral zone(as will be described hereafter) to provide edge lift at the peripheryof the lens and allow required tear flow under the lens. The desirededge lift preferably avoids excessive standoff, typically less than 100microns and no more than about 150 microns. Typically, the lens standoffis in the range of 40-60 microns. In other ways, lens 10 is similar toother corneal contact lenses, having a cross-sectional thicknessgenerally in the range of 0.05 to 0.5 millimeters or other suitablethickness, but being uniquely variable in thickness due to the“harmonic” correspondence between the front and back surfaces of thelens as will be described in more detail hereafter, along with theability to adjust the center and edge thickness relative to the othertwo. The lens 10 can be fabricated from any suitable contact lensmaterial, such as fluorosilicon acrylate, silicon acrylate,polymethylmethacrylate or another suitable material. Oxygen permeablematerials are preferred particularly when the lens is worn overnight topermit non-wear during the day.

The lens 10 in general comprises a lens body having a posterior surface13 including a central zone 12 provided with a curvature determined bythe reshaping to be imparted to the cornea of a patient for correctionof visual defects. The posterior surface 13 also comprises a firstannular zone 14 and a second annular or peripheral zone 26, eachpreferably being concentric with central zone 12. The central zone 12 isshown in more detail in FIG. 3, and is spherical in shape in thisembodiment. Alternatively, the central zone 12 could be aspherical,toric, or comprised of a combination of annular spherical and/oraspherical zones. In the example shown in the Figs., the surface isspherical and is defined as having a radius of curvature R₁. The radiusof curvature, R₁, may be chosen based upon characteristics of a patientseye for which the lens 10 is being designed, and particularly related tothe amount of correction required. To determine the refractive error ofthe eye of a patient, typical refractive measurements may be used and/orkeratometry measurements. Using a keratometer, a single point value forthe radius of curvature at the apex of the patient's cornea may bemeasured. Thus, there may be no need for complex corneal topographymeasurements, such as by use of photokeratoscopy or videokeratoscopytechniques, to design the lens. Selecting the design of the other zonesmay be accomplished by trial fitting the lens of the present inventionas an example, or in conjunction with a model eye. The objective of thetrial lens fitting, using a set of fitting lenses as will be describedin more detail, is to discern the sagittal depth of the cornea from itsapex to the diameter at which ultimate tangential touch would occur andto select an angle making tangential touch at a diameter appropriate forthe diameter of the treatment lens. Additionally, for some patients,other topographical knowledge of the cornea may be required. Thus, ifdesired, the corneal topography of the eye could be determined by use ofphotokeratoscopy or videokeratoscopy techniques as an example, tofacilitate determining the required reshaping for correction of visualdefects. Generally, for most patients the diameter of tangential touchby the second peripheral zone of the lens with the cornea is beyond thediameter measured by corneal topographers. Sagittal depth of the corneamay be grossly estimated by extrapolating data from corneal topographybeyond the range of measurement. Any suitable method for determining theamount of correction required or determining the corneal topography ofthe patient may be used. The central zone 12 is also defined as having achord diameter D₁ as shown in FIG. 3. The base curve provided on theposterior surface of central zone 12, although shown as spherical, mayalso be aspherical if desired, or have such other shapes as are known tothose skilled in the art to impart the desired shape to the cornea forcorrection of visual defects such as astigmatism or presbyopia. Ingeneral, the radius of curvature R₁ of the posterior surface 13 ofcentral zone 12 is equivalent to the desired post treatment radius ofcurvature for the cornea that is undergoing reshaping. Typically, an CRTcontact lens is to be fitted such that pressures exerted upon the lensduring lens wear are transmitted to the cornea, with the corneal tissueunderlying the portion of the lens applying pressure being effectivelyredistributed in a desired manner. For example, the central zone 12could be designed to correct presbyopia by reshaping the cornea, ordesigned to correct presbyopia without contacting the cornea, dependingagain on the characteristics of the needs of the patient. Theredistribution of corneal tissue causes the cornea to temporarily takeon the radius of curvature of the posterior surface 13 of central zone12 to provide correction of visual defects based upon the presenttopography of the patient's cornea. The intended effect of the CRT lens10 is to sphericize the apical corneal cap and establish a new radius ofcurvature for it. In the embodiment as shown in FIGS. 1-3, the posteriorsurface 13 of the central zone 12 of lens 10, being spherical, requiresonly the designation of the base curve and the diameter as shown in FIG.3. Fitting observations may be computationally translated to peripheralparameter choices. These choices provide that the peripheral designelements (connector zone depth, peripheral zone angle and overalldiameter) allow apical corneal contact, promote lens centration, a voidexcessive edge standoff and avoid pretreatment peripheral cornealengagement which would oppose pressure applied to the corneal apex bythe central zone, and possibly other characteristics.

In order to simplify fitting, as well as to allow adjustment of the lensdesign, the visualization of the lens fit to the eye, the ability toteach a fitter and communicate changes in the preferred lens geometryfor a given patient, as well as the ability to assess adjustments andthe fit of the lens, it is desirable to use the minimum number ofvariables describing easily visualizable geometric shapes in selectingand designing lenses. As will be seen in more detail as the lensgeometry is described, the invention is directed in part to a method offitting a contact lens, wherein the fitter can be provided with a lensset where the central zone 12 diameter, connecting zone 14 width, lensdiameter and edge profile have been predetermined by a manufacturer. Thecorneal curvature needed to eliminate refractive error and the centralcorneal curvature can be determined by typical refractive measurementand/or simple keratometry measurements, thereby enabling the lens designto be characterized by specifying the depth of the connecting zone 14and the angle of the peripheral zone 26 relative to the central axis ofthe lens 10. Minimizing the number of variables, as well as enablingadjustment of these variables without impacting the design or functionof the other zones, provides unique and extremely powerful fittingcapabilities. As an example, in this manner, the fitter may be providedwith a set of fitting lenses having a fixed depth for the connectingzone 14, with a series of base curves for the central zone 12 or with aset of fitting lenses with fixed base curve an a series of connectingzone depths to determine sagittal depth of the cornea at the point oftangency between the cornea and the second peripheral zone. Similarly,the fitter could be provided with a lens set with the sagittal depth ofthe base curve and connecting zone 14 fixed, having a fixed sagittaldepth greater than normal corneas and the angle of the peripheral zone26 varied in a series of fitting lenses to determine the desiredrelationship between these zones. A set of fitting lenses may beconfigured to have a plurality of visible concentric rings 27 on eitherthe posterior surface 13 as shown in FIG. 4A or the anterior surface 15as seen in FIG. 4B, allowing accurate determination of the lens diameterat which substantially tangential touch occurs between the lens and thecornea to determine the angle of the at least one peripheral zone. Inthe embodiment of FIG. 4, the rings 27 are formed during the manufactureof the lens by lathe, such as by forming grooves in the surface 13 or15. In the fitting procedure, fluoroscene may be used to assess therelationship of the lens to the cornea. The formation of the rings 27 asgrooves allows fluoroscene to accumulate slightly within the grooves,such that the fluoroscene highlights the ring allowing easier viewing ofthe lens diameter at which substantially tangential touch occurs betweenthe lens and the cornea. Alternatively, the visible rings may be formedin any other suitable manner, such as by laser etching, printing,embossing or any other suitable approach. The final lens design can thenbe selected by simple computation of the angle. Table 1 as followsillustrates the fitting lens sets contemplated according to theinvention.

TABLE 1 Plurality of rings on some Connector Peripheral or all of theSet # Base Curve Depth zone angle set lenses 1 V F F N 2 F V F N 3 F F VN 4 V F N Y 5 F V N Y 6 F F V Y Where F = fixed, V = variable, Y =present, N = not present

Fitting may be performed using set 1 with set 3, or set 2 with set 3, orwith set 4 alone or set alone, or set 4 or set 5 with set 6. Further,although the description above relates to determining certain parametersof the lens design, if desired, the other variables in the lens designof the invention could also be adjusted if desired, but limiting thenumber of variables which are adjusted in the fitting process mayprovide significant advantages.

For example, it has been observed that the flexibility of the rest ofthe design features of the lens according to the present invention makeit extremely rare that a diameter of the central zone other than 6 mmwill be required, such that this possible variable may be held constantwhile allowing a fitter to properly design an appropriate lens for agiven patient. At the same time in cases of patients with highrefractive error, low corneal eccentricity, hyperopia, narrowing thisdiameter and expanding the connecting zone width can avoid deep tearzones under the lens that might cause bubbles as will be described inmore detail hereafter. Thus, it is possible to provide a lens designwith design variables minimized, and yet to allow suitable designs forsuch rare cases as this are enabled. The variables which are possiblymodified to achieve particular characteristics are available to thefitter, but also may be held constant for a variety of lens designs tosimplify the fitting process. In this example, both variables areexcluded from those used in normal fitting. The benefit to excludingdesign variables may outweigh their utility for many lens designs, andagain simplifies the design and fitting process. As an example, thedesign may accommodate nearly every patient using only four variables,with correction provided for nearly any eye. These variables include thebase curve, connecting zone depth, peripheral zone angle and overalldiameter. Two of these (Base curve and Diameter) are so firmly dictatedby needed correction, corneal curvature, and HVID, that they can be inmost cases fixed for a patient from records of normal eye exam. Hence,the proper fitting and design of the lens may require only two variables(connector depth and peripheral zone angle) and these are easilydetermined, visualized and discussed between fitter and manufacturer orconsultant.

Thus, the design of the central zone 12 is simple and easily configuredto produce the desired reshaping of the cornea based upon the patientsmeasured characteristics. In general, the base curve is generallydetermined by those skilled in the art of orthokeratology atapproximately 0.2 millimeters greater in radius of curvature than thepresent corneal shape, for each diopter of myopia that is to becorrected. Other visual defects may require different configurations toresult in the desired reshaping. Based upon typical patient populations,and refractive errors generally found in human eyes, the radius ofcurvature R₁ may be in the range from 5 millimeters to 12 millimeters,or more typically in the range from 6.8 millimeters to 10 millimetersradius.

The other parameter of the central zone 12 relating to chord diameter D₁is generally fixed at 6 mm, but in cases where it is necessary to changethis variable, it is determined by correlation to the frill pupildiameter of the patient, as measured under dark conditions. Such adesign rule is not required in the present invention, and it may beeasier to achieve large visual defect correction with smaller diametercentral zones 12, and thus the relationship of the chord diameter D₁ tothe pupil diameter may vary. In a particular situation, such ashyperopia or very high myopic corrections, such as above 5 diopters, itmay be acceptable to achieve the visual defect correction desired, touse smaller diameter central zones 12. This still may be acceptable eventhough under low lighting conditions, some flare or visual aberrationsmay be experienced. In general, the chord diameter D₁ is in the rangefrom 2 to 10 millimeters, and more typically 3.5 millimeters to 8.5millimeters. Thus, once the corneal characteristics and/or topography ofa patient is determined, the design of the central zone 12 may beconfigured to impart the desired amount of pressure to the cornea forreshaping and redistribution of corneal tissue. Thus, the posteriorsurface 13 of central zone 12 is designed with particular attributes,while the characteristics of the anterior surface 15 of central zone 12are of less significance. The front surface 15 of the lens 10 couldtherefore be configured to be similar to the geometry of standardcontact lenses with or without lenticulation. As examples, the anterioror front surface 15 of lens 10 may be configured from contiguousspherical surfaces, contiguous aspherical surfaces, toric surfaces orcombinations thereof. It is also possible and may be preferable todesign the front surface of the lens 10 to mirror or be substantiallythe same shape as the posterior or back surface 13 using identicaldesign techniques. In the embodiment as shown in FIGS. 1-2, The anteriorsurface 15 of the lens 10 is made to mirror the back surface, such thatthe anterior surface 15 exactly parallels the posterior counterparts ofthe lens with equal spacing between anterior and posterior surfaces fromthe center to the peripheral edge of the lens 10. At the same time, itis also possible to design the front surface to mirror the back surface,but to do so with different spacing between the surfaces from the centerto the peripheral edge of the lens as will be described in more detailwith respect to an alternative embodiment.

It may also be desirable to impart the lens 10 with a desired opticalpower based upon the patient's vision characteristics. In this regard,the anterior surface 15 may be configured in combination with thecomputed posterior base curve of lens 10 to impart the desired opticalpower to lens 10. It is however normally true that the base curve whichwill yield the intended correction compensates fully for any necessaryoptical correction needed by the patient thus the optical power for alllenses offered can be set a single value near plano. Typically a basecurve which would provide a correction slightly greater than required isemployed and a power just slightly deviating from plano to compensate isprovided in the lenses.

Turning now to FIG. 5A, a schematic illustration of the annularconnecting zone 14 is shown. The connecting zone 14 is adjacent andconcentric to the central zone 12, and in general is designed to have asigmoidal configuration, or the shape of an elongated “S” which isrotated circumferentially about central axis 15 through space at aradius R₂ associated with the middle of the sigmoidal form or transitionfrom a concave curvature to a convex curvature in the connecting zone14. The connecting zone 14 is a surface of rotation about the centralaxis of the lens 10, with a beginning point defined as a circlecircumscribed by the upper limit of the rotated sigmoidal shape having adiameter D₁, at 16, and terminating at a bottom point defined as acircle with a diameter D₂ circumscribed at the bottom of the rotatedsigmoidal curve at 18. The connecting zone 14 has a depth Z₁ equal tothe distance from the plane containing the upper circumscribed circle at16 and the plane containing the lower circumscribed circle at 18,measured parallel to the central axis. The end points of the connectingzone 14 (associated with circles 16 and 18) represent junctions to thecentral zone 12 and peripheral zone 26 respectively. As previouslymentioned, the anterior surface 15 of the lens 10 may be made to mirrorthe posterior surface, such that the location of junction points, whichmay be referred to as J1 and J2, are selected by translating radiallyfrom the analogous junctions on the posterior surface. The translationof J1 and J2 are individually chosen and determine the separation of theanterior and posterior surfaces and along with the front curve radiusand peripheral zone angle define the equations for the profile of theanterior connecting zone.

Turning to FIG. 5B, the design of the connecting zone 14 may be betterunderstood with reference to a semi-meridian section corresponding tothe zone 14, inscribed within a rectangle 20, and how it may bedesigned. The characteristics of connecting zone do not alter those ofthe central zone 12 or the second annular zone 26, and thus a lensdesigner or fitter can design the dimensions of the inscribing rectangleindependently and adjust as necessary without affecting the design of anadjacent zone. As should be understood with reference to FIG. 2, theconnecting zone may be designed such that connecting zone 14 smoothlyjoins the central zone 12 at the beginning point 16, corresponding totop left of rectangle 20. Similarly, the connecting zone 14 may be madeto smoothly join the peripheral or second annular zone 26 at point 18,at the bottom right corner of the rectangle 20. The primary designconsideration for the connecting zone 14 relates to defining theposterior surface 13 of zone 14 to begin at a point coinciding in spacewith the periphery of the posterior surface of the central zone 12, andterminating at a point in space where the posterior surface of theperipheral zone 26 begins. In designing lens 10, the design of centralzone 12 will dictate the beginning point 16 and the slope associatedwith the connecting zone 14 at this point. Once this position and slopeare located on a meridian section of the posterior surface of lens 10,the width and length of the imaginary rectangle 20 may then be set suchthat connecting zone 14 terminates at position 18 corresponding with thebeginning of the posterior surface of the peripheral zone 26 to bedescribed hereafter. The design of the meridional profile of connectingzone 14 thus may be defined or described by its axial length andhorizontal width. Changes to the shape of the sigmoidal curve thereforeonly relate to the location in space of the central zone 12 andperipheral zone 26 and the slopes at the points of connection to thesigmoidal curve, but do not define or reflect upon the shape of thesezones, and any design parameters for these zones would be acceptable. Inthis way, the design of the connecting zone 14 allows a fitter or lensdesigner to freely determine the location of the central zone 12relative to the peripheral zone 26, with the connecting zone 14 thenmatched to correspond to such locations.

The design of the connecting zone 14, being a sigmoidal curve in thisembodiment, also assures the maximum “void space” geometry imparted bythe sigmoidal shape of the curve as it relates to positioning on thecornea of a patient. Additionally, providing the ability to design theconnecting zone 14 based upon the beginning and ending point 16 and 18respectively, allows the fitter to easily visualize this element of lens10 as a simple rectangle and ultimately easily visualize any changesmade in the design of this zone. The ability to more easily visualizehow changes in the design will impact proper fitting to the patients eyeand cornea, makes it easier to properly select parameters to achievegood fit more simply and efficiently. As the characteristics and designparameters of the connecting zone 14 are independent of the designparameters of central zone 12 or peripheral zone 26, judgement aboutdesign parameters of the connecting zone 14 are not complicated bypossible effects on the shapes or function of the other zones. In thepreferred embodiment, the design parameters of the connecting zone 14are desired to only affect the relative location in space of the otherzones 12 and 26.

In order to achieve comfort in wearing the lens 10, it is also a featureof the embodiment as shown in the Figs., that the connecting zone 14 bedesigned such that the slope of the sigmoidal curve at the points ofintersection 16 and 18 with the central zone 12 and peripheral zone 26respectively, be substantially or exactly matched to the slopesexhibited at these locations in the zones 12 and 26. The meridionalprofile of the connecting zone is thus shaped to substantially match theslopes of the central zone 12 and peripheral zone 26 on adjacent sides.As shown in FIG. 7, the matching of the slopes of the sigmoidal curve atthe points of intersection 16 and 18 eliminates the need for any curveblending or manual curve fitting between independent, non-continuoussurfaces as in the present invention. Such curve blending ifaccomplished by means of polishing imparts an unknown and indescribableand irreproducible shape to this important region of the lens and maywell affect to optical quality originally imparted to the central zoneby precise lathing. If blending is accomplished by manual curve fittingit is not possible to automatically compute a lens design for aparticular eye but rather requires a designer to make unique choices forevery individual greatly slowing the delivery and increasing the cost ofthe lens. Further, the sigmoidal shape of the posterior surface ofconnecting zone 14, and the matching of the slope to that of centralzone 12 at 16, tends to create the desired void space between the lensposterior surface and the pretreatment cornea, at the position ofsurface 14 adjacent central zone 12. The void space between theposterior surface of the lens and the pretreatment cornea is initiallyfilled with tears and allows redistribution of corneal tissue therein.In the present invention, the connecting zone 14 is designed to providea desired amount of void space at the desired location.

In prior art orthokeratology lens designs, the use of “reverse curves”,several indicated by reference numeral 22 in FIG. 5C can create too muchvoid volume at this location, leading to bubbles between the lens andcornea, or too little room or void space for proper displacement ofcorneal tissue. Further, it should be noted that such “reverse curves”require at least a radius and origin be specified before the location ofthe upper left and lower right corners can be computed. The difficultyof this computation and the consequent lathe set up preclude mostfitters and lab operators from defining these locations except by trialand error, and that the sharp junctions created require such intensepolishing that even if the intended locations were designated they couldnot be guaranteed or located in the final product. Providing rationallyspaced continuum of lenses using “reverse curves” to meet thebiodiversity of human corneas cannot be accomplished without the use ofnearly intractable parameter spacing relationships, and is so difficultthat manufactures are forced to offer a restricted range of lenses whoseparameters give little insight into the fitting characteristics and noguidance on how to improve a misfitting lens. The sigmoidal curve designof the connecting zone 14 according to the present invention, provides amajority of the void volume at the location it is needed near junction16, while allowing matching slopes to be attained to avoid curveblending. Any use of “reverse curves” or radial arcs 22 would at bestdistribute any void volume uniformly throughout the reverse zone whichis not optimum. Based upon the possible creation of bubbles or the like,the volume created between the lens and the cornea must be considered,and would limit the choice of reverse arcs available for use. Thus, thesigmoidal curve design according to the present invention provides thedesired attributes without limiting the designer as the use of reversearcs would, and since the sigmoidal curve is mathematically defined (seebelow) by the choice of the depth (and possibly the width) of thedefining rectangle 20, this eliminates any need for the designer toparticipate in its designation beyond specifying its depth while beingassured it is of optimum ultimate configuration. The prior art use of“reverse curves” has also taught that such “reverse curves” are ofshorter radius than the base curve or other adjacent arcs in the Ortho-Klens design, such as in alignment or anchor zone arcs at the peripheryof the lens. In the present invention, the sigmoidal curve design is notlimited in this respect, and provided that the origin of the radius R₂not be restricted to be on the axis of the central base curve 12, theconnecting zone 14 may be provided with effectively flatter geometry'sthan the adjacent central zone. Additionally, the computed meridionalprofiles of the zones 10, 12 and 26 may be different at different anglesof rotation about the lens central axis allowing non-rotationallysymmetric designs visualizable in each meridian in the same manner as isused for one meridian in a rotationally symmetric design. Moderncomputer controlled lathes can easily cut flatter radii in this zone ifdesired. Although a reverse curve 22 provided with a flatter radii inthe zone may resolve in part the problems of creating the desired amountof void volume adjacent the central zone, the void volume is stilluniformly distributed as discussed above, and the use of continuousposteriorly concave arcs would also leave at least one junction withother portions of the lens which would require manual curve blending.The mathematically described sigmoidal shaped curve of the connectingzone 14 alleviates these problems and simplifies design of the lens.

The posterior surface 13 of the connecting zone 14 is determinedmathematically once various parameters are measured or determined for agiven patient's condition and the desired reshaping to be imparted tothe cornea. Again, it is desired that the connecting zone 14 interact toproperly position the peripheral zone 26 at a desired position relativeto the central zone 12, as will be hereinafter further described. Thus,the inputs for determining mathematically the curve in the connectingzone 14 will include the base curve of the central zone 12, which againmay be spherical and defined by a radius (r_(B)) as well as the slope(M) of the peripheral zone 26. The connecting zone may further bedefined by inputs including the length or “depth” of the sigmoidal curve(L), the radial distance from the center of the lens to the base curvejunction with the sigmoidal curve (J₁), the radial distance from thecenter of the lens to the junction of the sigmoidal curve with theperipheral zone, and the width of the sigmoidal curve (W) computed bysubtracting (J₂) from (J₁). In this embodiment, the equation for thesigmoidal curve isy _(s) :=A·x ³ +B·x ² +C·x+D  (Eq. 1)

Using the above inputs, various intermediate results may then bedetermined to yield a design of the lens which can be visualized withrespect to fitting properly for the patients eye and treatment desired,allowing the fitter to more easily vary certain parameters to adjust andproperly fit the lens. As each zone of lens 10 is a surface of rotation,the lens design may be visualized with respect to a transmeridiansection, such as shown in FIG. 12. The embodiment shown in FIG. 12 showsthe thickness of the lens 10 varying from center to edge. For aparticular patient, a designer may prefer to provide the central zone 12having mirrored front and back surfaces, but thin, and the peripheralzone 26 mirrored but thicker. In such an embodiment, the transitiontakes place in the connector zone 14, which mirrors but graduallychanges thickness. Using the above information, the Y value for thejunction (J₁) between the central zone 12 and connecting zone 14 isdefined by the equationy _(j1):=√{square root over (r _(b) ² −J ₁ ²)}  (Eq. 2)

The X value for the junction (J₂) between the connecting zone 14 andperipheral zone 26 is defined by the equationx _(j2) :=J ₁ +W  (Eq. 3)

While the Y value for the junction J₂ is defined by the equationy _(j2) :=y _(j1) −l  (Eq. 4)

With these values in hand one can compute the coefficients A, B, C, Dfor the equation of the sigmoidal curve in this same coordinate system.

The values for the coefficients A, B, C and D of equation 1 are definedby equations 5-8 as follows:

$\begin{matrix}{{A:=\frac{- \left\lbrack {{\frac{- 1}{\left( {{2 \cdot J_{1}} - {2 \cdot x_{j\; 2}}} \right)} \cdot M} - {\frac{1}{\left\lbrack {\left( {{2 \cdot J_{1}} - {2 \cdot x_{j\; 2}}} \right) \cdot \sqrt{r_{b}^{2} - J_{1}^{2}}} \right\rbrack} \cdot J_{1}} + {\frac{1}{\left( {J_{1}^{2} - {2 \cdot J_{1} \cdot x_{j\; 2}} + x_{j\; 2}^{2}} \right)} \cdot J_{1} \cdot M} + {\frac{1}{\left( {J_{1}^{2} - {2 \cdot J_{1} \cdot x_{j\; 2}} + x_{j\; 2}^{2}} \right)} \cdot y_{j\; 2}} - {\frac{1}{\left( {J_{1}^{2} - {2 \cdot J_{1} \cdot x_{j\; 2}} + x_{j\; 2}^{2}} \right)} \cdot x_{j\; 2} \cdot M} - {\frac{1}{\left( {J_{1}^{2} - {2 \cdot J_{1} \cdot x_{j\; 2}} + x_{j\; 2}^{2}} \right)} \cdot \sqrt{r_{b}^{2} - J_{1}^{2}}}} \right\rbrack}{\left\lbrack {{\frac{- 3}{\left( {{2 \cdot J_{1}} - {2 \cdot x_{j\; 2}}} \right)} \cdot J_{1}^{2}} + {\frac{3}{\left( {{2 \cdot J_{1}} - {2 \cdot x_{j\; 2}}} \right)} \cdot x_{j\; 2}^{2}} + {\frac{1}{\left( {J_{1}^{2} - {2 \cdot J_{1} \cdot x_{j\; 2}} + x_{j\; 2}^{2}} \right)} \cdot J_{1}^{3}} - {\frac{3}{\left( {J_{1}^{2} - {2 \cdot J_{1} \cdot x_{j\; 2}} + x_{j\; 2}^{2}} \right)} \cdot J_{1} \cdot x_{j\; 2}^{2}} + {\frac{2}{\left( {J_{1}^{2} - {2 \cdot J_{1} \cdot x_{j\; 2}} + x_{j\; 2}^{2}} \right)} \cdot x_{j\; 2}^{3}}} \right\rbrack}}{B:=\frac{- \left( {{A \cdot J_{1}^{3}} - {3 \cdot J_{1} \cdot A \cdot x_{j\; 2}^{2}} + {J_{1} \cdot M} + y_{j\; 2} + {2 \cdot A \cdot x_{j\; 2}^{3}} - {x_{j\; 2} \cdot M} - \sqrt{r_{b}^{2} - J_{1}^{2}}} \right)}{\left( {J_{1}^{2} - {2 \cdot J_{1} \cdot x_{j\; 2}} + x_{j\; 2}^{2}} \right)}}} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$C:=−3·A·x _(j2) ²−2·B·x _(j2) +M  (Eq. 7)D:=y _(j2)+2·A·x _(j2) ³ +B·x _(j2) ² −x _(j2) ·M  (Eq. 8)

Inserting these values of A, B, C, D into Eq. 1 for the sigmoidal curveand solving the equation over the range of x values from the firstjunction J1 to the second junction J2 yields the location of all pointsalong the curve in coordinates usable by modern computer controlledlathes such as the Optoform 50. Again the posterior surface 13 isdefined to have particular attributes, while the design of the anteriorsurface can mirror the posterior shape or may be shaped as in a typicalcontact lens. Specifically the relationship of a mirrored anteriorsurface is easily determined by locating the corners of the anteriorsigmoid defining rectangle in reference to those of the posteriorsigmoid defining rectangle. The anterior peripheral zone departs fromthe lower corner of the anterior defining rectangle with the same slopeas its posterior counterpart and thus remains parallel, at least untilit meets the edge zone of the peripheral zone 26 is reached, which mayinclude edge lift characteristics, as will be hereinafter described inmore detail. The anterior central zone departs from the upper corner ofthe rectangle with the radius of curvature required to yield theappropriate lens power. Since this power is usually near plano in CRT(the tear lens gives the necessary power correction), the thickness atthe starting point essentially equals that at the lens center, thusplacing no constraint on the lens design when selecting the relationshipbetween the posterior and anterior defining rectangles with followingexamples indicating this relationship as being variable.

Turning now to FIG. 6, a peripheral or landing zone 26 according to anembodiment of the invention is shown in more detail. The peripheral zone26 in general is a second annular zone adjacent and concentric to thefirst annular or connecting zone 14, and thereby is connected to thebase curve 12 by means of the connecting zone 14. The peripheral zone inthis embodiment is formed as a truncated conoid which may be uncurvedover at least a substantial portion thereof. The conoid may be definedby the diameter of the upper limit of the truncated conoid, being D₂,corresponding to the diameter of the bottom limit of the rotatedsigmoidal curve forming the connecting zone 14. The peripheral zone 26may further be defined by the diameter of a bottom limit of thetruncated conoid at D₃. The angle that the interior or posterior surfaceof the conoid makes with a plane containing the central axis of lens 10may be defined as the angle of the conoid, which is chosen by the lensdesigner or fitter. The relationship of the meridional profile of theperipheral zone 26 to the meridional profile of the connecting zone 14may thus be described by the angle the meridional profile of theperipheral zone 26 makes with the a line perpendicular to the centralaxis of the lens 10. In the design of the peripheral zone 26, the properdesign for a given patient's cornea and the amount of redistribution ofcorneal tissue which is desired by the CRT treatment, will in turndictate relatively precisely the design of the peripheral zone 26. Themeridional profile of the peripheral zone 26 may be described by theangle it makes with a line perpendicular to the central axis of thelens, its curvature and its extension from the central axis of lens 10.The peripheral zone 26 is again separate and independent from thecentral zone 12 and connecting zone 14, and altering its design ingeneral does not effect the design of these other zones. At the sametime, depending upon the characteristics of the central zone 12, theperipheral zone is designed to work in cooperation therewith in aparticular manner in the preferred embodiment of the lens.

In a properly fit lens at the beginning of an CRT treatment procedure,the central zone 12 is designed to contact the cornea at its apex, andimpart a desired amount of pressure on the cornea depending upon theamount of correction desired. Similarly, the length of the connectingzone 14, as represented by the length of the imaginary rectangle asshown in FIG. 5B, as well as the angle of the truncated conoid formingthe peripheral zone 26, are selected such that the peripheral zone 26 isgenerally elevated above the peripheral corneal surface a desiredamount. The amount of elevation is generally selected such that theperipheral zone 26 does not engage the cornea until the central corneahas been sufficiently redistributed to yield the desired final cornealshape or change in the spherical radius of the cornea. Generally, about3 to 7 microns of elevation above the cornea at the tangent point forevery diopter of correction sought may be acceptable. It should also berecalled that there is a thin layer of tear (5-6 microns deep) which isbelieved to be non-displaceable from the corneal surface even by CRTpressures. As corneal tissue is redistributed, and at a later point inthe progression of the process, the design of the peripheral zone 26tangentially engages the curved cornea in a predetermined manner, forexample approximately half way between the lens edge and J2, or slightlynearer the lens edge. By selecting the extension of the peripheral zoneand its angle, the fitter is able to place the location of thetangential touch at the greatest corneal diameter possible. Due to thegreater slope of the cornea at the larger chord diameters relative tothe central cornea, the ratio of approach to the cornea at the peripheryis only a fraction of that observed at the corneal center. Employing thewidest diameter possible for the ultimate tangential touch allows theelevation of the lens peripherally to be at a minimum relative to thedisplacement necessary centrally for refractive correction. Havingminimized the peripheral elevation improves lens comfort and centration,thus allowing greater consumer satisfaction and larger corrections. Asfurther distribution of corneal tissue occurs, and the engagement withthe peripheral zone 26 increases, the compressive force of the lens onthe cornea is borne in a progressive amount by the peripheral zonespreading equally in both directions from the original point oftangential contact 26, until the counteracting force imparted on thecornea by the peripheral zone 26 grows to effectively neutralize thecentral compressive forces imparted by the base curve 12. In thismanner, an equilibrium corneal shape is established, with the peripheralzone 26 contributing to the equilibrium achieved while assuring thatneither the lens edge nor the connecting zone can dig into the cornea.

In rare cases, the desired correction to be imparted to the cornealshape is significant, such that to achieve the full desired correction,the pre-correction elevation of the peripheral zone 26 would be so greatthat wearing the lens would create discomfort, or lead to dislocation ordecentering of the lens from the desired location on the cornea. In suchan instance, the correction imparted by the lens 10 may be performed instep-wise fashion, with each step requiring a lens of a similar designto that described, but designed for only partial correction of thecorneal shape before equilibrium is achieved by means of the peripheralzone 26. Subsequent lenses in the step-wise series would thereafter takeup where the preceding lens terminated in terms of redistribution ofcorneal tissue, to continue to process. The step-wise approach wouldcontinue until the desired correction was fully achieved. In such aprocess, parameters of subsequent lenses in the step-wise series couldin effect remain the same except for shortening the length of thesigmoidal curve (within its imaginary rectangle) as described withreference to FIG. 6, to in turn provide the desired pre-correctionelevation of the peripheral zone 26 relative to the partly reshapedcornea.

Turning now to FIG. 7, an edge portion of the peripheral zone 26 isshown schematically, and is specially designed to provide comfort andproper function in a contact lens for normal wear or in CRT process. Asshown in FIG. 7, the edge of a lens according to the invention may beprovided with a smoothly contoured profile, which is facilitated by thegenerally parallel relationship between the interior and posteriorsurfaces in the peripheral zone 26, as well as by the straightness ofthe peripheral zone 26 itself in this embodiment. The edge terminus ofthe peripheral zone 26 is shown in cross-section in FIG. 7, and adividing line bisecting the lens at this region may be envisioned. Sucha dividing line may be envisioned to be substantially parallel to andbetween the anterior and posterior surfaces as shown at 150. Thisimaginary dividing line may be nearer to the anterior or posteriorsurface in the edge region. In the embodiment as shown in FIG. 7, thisline may be positioned in closer relationship to the posterior surface.A designer can then imagine a quadrant of an ellipse whose center is onthis dividing line and whose long axis would extend from the selectedellipse center along the chosen dividing line, to just reach the veryedge of the lens. The short axis of the ellipse would extendperpendicularly to the long axis from the center point to contact one orthe other sides of the lens. As shown in FIG. 7, the short axis extendstoward the posterior surface meridional profile to intersect theposterior profile. Where the short axis reaches the edge, the ellipsewould parallel the edge, and where the long axis reached the tip, theellipse would have curved so as to be perpendicular to the edge at thepoint where it meets the dividing line. The profile of the quadrant ofthe ellipse thus merges smoothly with the profile of the peripheral zone26 and replaces that portion of the meridional profile of the peripheralzone 26 in that region beyond the intersection of the short axis and theprofile of the peripheral zone 26 to become the profile of the posteriorlens terminus. This posterior terminus can join a similar but invertedstructure on the profile of the anterior surface of the lens at theultimate tip of the lens to form a smooth junction. The dividing linebeing chosen to be at a location 10 to 90% of the thickness of the lensfrom the posterior to the anterior and the long axis of the ellipsechosen to be about 0.01 mm to 2.0 mm in length.

A “mirror image” of this ellipse may be imagined on the other side,joining the anterior cross-sectional edge to the tip. The apices of theellipses would necessarily meet at the dividing line 150, at the tip,and each would roll back to parallel an adjacent edge of the lens. Whenthe dividing line is not midway between the anterior and posteriorsurfaces, the ellipse quadrants would be differently shaped, but wouldalways smoothly join each other at the tip, and roll smoothly back tomeet the original cross-sectional edges. The two ellipse quadrants neednot have equal long axis, although their short axis are defined by theplacement of the original dividing line 150. By manipulating thelocation of the dividing line between the anterior and posteriorsurface, such as by a fraction of this thickness, and manipulating thelengths of the long axis of the ellipse quadrants in each ellipse, onecan achieve mathematically and geometrically a desired edge shape. Adesired edge shape is then easily cut by means of computer controlledlathes or the like. Altered edge configurations may be better suited toa particular patient, to better accommodate patient characteristics,such as lid aperture and tightness, as well as high amounts ofperipheral astigmatism in the corneal shape. The ability to alter theedge configuration again makes the lens according to the inventionflexible and adaptable to a particular patients needs, while providing asimple design which is more easily fitted to the particular patient. Inexamples as will follow, in many cases, the edge configuration suitablemay include using equal long axis of 0.4 mm, with a dividing line 150 atapproximately 25% of the way between the posterior and anterior surfacesof the lens. Another example as will be seen hereafter, provides analtered edge configuration for use with a very large diameter lens,which includes a thick peripheral zone, for use on corneas with higheccentricities. In such a special case, a long axis of 2 mm with adividing line at 45% of the distance between the posterior and anteriorsurfaces may be desirable. It should also be recognized that the

anterior profile of the peripheral zone can be designed to have anequivalent angle to that of the posterior profile in the peripheralzone. The anterior lens terminus will have an elliptical curvaturederived from a quadrant of an ellipse and extending to the intersectionpoint with the posterior surface.

Turning now to FIGS. 8A-8C, in the design of the peripheral zone 26, theangle of the conoid and its chord diameter D₃ are preferably chosen sothat upon engagement with the cornea after redistribution of cornealtissue as described above, the first point of contact of the cornea withthe peripheral zone 26 is approximately midway between the junction (J₂)of the peripheral zone 26 with the connecting zone 14 and the outsideperipheral edge of the lens 10, but slightly nearer the peripheral edge.This engagement is shown in FIG. 8A for a portion of zone 26 on cornea30. In this manner, the chance of “toe down” or “heel down” engagementof the peripheral zone 26 with the corneal surface, as shown in FIGS. 8Band 8C respectively, is minimized. Such heel down or toe down engagementcan lead to corneal abrasion or other complications, and in general,creates undesired discomfort and minimizes the extent of achievablecorrection. These conditions are easily avoided even after treatmentcompletion when the non curving (infinite radius) peripheral zone isemployed, but which are common or mandatory with common ortho K lensdesigns which use cornea facing concave curvatures with radii less than17 mm. It remains unrecognized by those skilled in the art that muchflatter radii are desirable, as historically making such curves withlathes designed to cut radial arcs has been very difficult.

As described above, the peripheral zone 26 is designed to be elevatedfrom the cornea at initial stages of treatment, with first engagement ofthe cornea with zone 26 occurring after a predetermined amount ofcorneal tissue redistribution. After first engagement, furtherredistribution of the central cornea will lead to further engagement bythe flat peripheral zone 26. Further engagement with the peripheral zonewill result in symmetric spreading in the width of the engagement zoneabout the point of first engagement, which ultimately will deteradditional corneal flattening while still avoiding heel down or toe downengagement.

In general for a CRT procedure, a designer may start with a diameterapproximately 1 mm smaller than the (HVID). The HVID gives a goodestimate of the total size of the cornea. The designer then can find theangle whose point of tangency is half way to two thirds of the way fromJ2 at 8 mm (the sum of standard central zone width (6 mm and connectorzone width 1 mm (2 mm considering both sides)) and the overall diameter.The lens base curve is computed from central keratometry and thecorrection required so all that is left is to use one method or anotherto determine the rectangle depth that leaves the tangent point elevatedabove the cornea approximately 3 to 7 microns per diopter of neededcorrection. The angle of the truncated conoid forming the peripheralzone 26 in the embodiment as shown is determined to assure that theultimate engagement of the peripheral zone 26 will be sufficiently farfrom the junction J₂ with connecting zone 14 to avoid toe downengagement. This determination may be made by modeling of the patientscornea and a lens designed in accordance with the present invention tovisually or mathematically determine the point of engagement uponcorneal tissue redistribution, or could be determined by trial fitting.With the angle of the conoid determined, it is possible then to selectthe final diameter of the lens, which in general is selected in trialfitting by noting the diameter at which a flat surface will deviate fromthe corneal surface sufficiently to yield the lens designer or fittersdesired “edge lift”. The edge lift of the peripheral zone 26 is theregion at the periphery of the lens which is generally required toassure good tear flow under the lens, and to more closely approximatethe corneal shape when excursions are made beyond the limbus. This willavoid abrasive interaction between the periphery of rigid lenses and thecorneal and scleral surface. This edge lift is common to all rigid lensdesigns as is well known to those skilled in the art of rigid lensdesign and fitting and is described in texts relating to rigid lensfitting. Beginning at the tangential contact point of the peripheralzone 26 with the redistributed corneal tissue, and extending theperipheral zone 26 outward, the posterior lens surface deviates furtherand further from the cornea generally. In the lens design, the diameterwill normally be set at a value where this deviation between lens andcornea is estimated to be sufficiently great to allow required tear flowunder the lens. It should be recognized that the simplicity of selectinga proper diameter for the lens 10 according to the invention for a givenlens or patient provides a significant advantage to the lens designer.As the posterior surface of the truncated conoid effectively provides aflat surface relative to the curved surface of the cornea, the properdiameter to achieve the contact between the peripheral zone 26 andcornea as described above is relatively simple, as compared to use of acurved arc. With a curved arc, estimating the location of the firstengagement with this arc and the curved cornea is nearly impossible, andeven further, estimating the edge lift at a given diameter is alsodifficult if not impossible to determine. Thus, providing the peripheralzone 26 in accordance with this embodiment of the invention simplifiesthe design process, and makes proper fitting of the lens easier and morecost effective.

The design of the peripheral zone 26 is also beneficial to the lensdesigner in other respects. In some patients, the lens design may besusceptible to decentering, and the fitter will look to stabilize adecentering lens using a larger diameter. Using a peripheral zone 26which is non-curved, the angle of the conoid of the peripheral zone 26may simply be steepened to move the contact point of peripheral zone 26with the cornea further from the junction with connecting zone 14allowing a larger diameter to be used without excessive edge lift.Again, because the zones of the inventive lens are separate and distinctfrom one another, the ability to steepen the angle of the peripheralzone 26 is provided without effecting the design elements of the otherzones. Again this process is easily visualized by a lens designer orfitter to arrive at an acceptable design more easily than in prior artlens configurations. Designing the peripheral zone 26 in accordance withthe principals of the invention provides a lens design which eitherinitially or in some cases a final lens of a treatment series, providesa lens which may be continuously used by a patient as a “retainer” lens.For such a purpose, the “retainer” lens must in its final engagementconfiguration perform as a typical rigid contact lens would, with regardto tear flow, centration and non-abrasive contact. Providing theperipheral zone to tangentially engage the curved cornea in the finalengagement configuration assures this relationship after redistributionof the corneal tissue is completed.

Several examples are set forth below.

Example 1

This example is based upon a patient having a prescription as follows:

1. 44.50×46.00@180, Rx−4.00−0.75×180, e=0.5, HVID 11.6

Based upon the prescription of patient 1 above, a lens designer wouldselect the power of the lens to correct the patient to a desired degree.In this example, with reference to FIG. 10, the lens/cornea powerdifference wanted for patient 1 is selected as −4.5 diopters asindicated at 200. The power difference selected is slightly more thanthe degree of correction required for the patient, to allow substantialcorrection of visual acuity over a longer period of time after reshapingof the cornea using the lens according to the invention. Patient 1 alsohas a fairly high astigmatism of 0.75, and the ellipticity of the corneais 0.5 as shown at 202. Patient 1 has an HVID of 11.6, as noted at 204,and based upon this HVID, the diameter of the lens which is recommendedis 10.6 mm as indicated at 206, being 1 mm less than the measured HVID.The selected diameter for the lens is shown at 209, being chosen basedupon the recommended diameter and the relationship between the zones ofthe lens. Based upon the lens/cornea power difference wanted, a basecurve of 8.4 is selected for this patient as indicated at 208. Otherparameters of the lens may be selected which provide desiredcharacteristics for most patient conditions, including the radialdistance from the lens center to the junction between the base curve andthe sigmoidal curve, indicated at 210 as 3.0 in this example. The widthof the sigmoidal curve is selected to be 1.0 mm as indicated at 212, thelens power is selected at 214, representing examples of variables whichcould be used to modify characteristics of the lens, but to simplifylens design, may be held constant to limit the number of variables usedby the lens designer. Based upon the desired correction, it waspreviously mentioned that the peripheral zone is initially elevatedabove the cornea to a predetermined degree to provide for redistributionof corneal tissue to a predetermined shape. The angle of the peripheralzone 216 is selected to be −35 to provide the proper relationship to thecornea as to spacing and to avoid toe down or heel down engagement aspreviously described. As an example, the elevation of the peripheralzone above the cornea can be selected at six microns per diopter ofcorrection to be imparted by the base curve. As indicated at 218, therecommended depth of the sigmoidal curve for providing the desiredelevation of the peripheral zone is 0.51 mm. Based upon these factors;the lens designer can note the relationship of the surface visually,such as shown in FIG. 10, representing the elevation of the posteriorsurface from the cornea along a semi-chord diameter of the lens. Asseen, the base curve 250, the sigmoidal curve 252 and the peripheralzone 254 are shown in relationship to one another, and allow thedesigner to select the proper diameter of the lens. To achieve propertangential contact between the peripheral zone and the cornea uponredistribution of corneal tissue as previously described, the positionof the peripheral zone can be determined, along with its angle, allowingthe fitter to vary the depth of the sigmoidal curve to obtain the properrelationship to the cornea. Variations of the depth of the sigmoidalcurve will in general move the peripheral zone curve 254 up and down inthe graph of FIG. 11, while changing the angle of the peripheral zonewill move this surface right or left. It should be evident that therelationship of the surfaces relative to each other and to the corneaare easily visualized and facilitate simple and proper design of thelens for a particular patient. It is also possible to show theindividual and cumulative volumes under the lens according to aparticular design of this type, such as shown in FIG. 12, relative to amodel cornea. This representation of the lens design provides thedesigner with an easy tool for identifying any void spaces, which maylead to the creation of bubbles beneath the lens. A semi-meridiansection of the lens according to this example is shown in FIG. 13, andit is noted that the thickness of the central and peripheral zonesremain substantially constant, with the sigmoidal curve thicknesstransitioning between the two, as shown in FIG. 15 Due to the relativelyhigh astigmatism of patient 1, the thickness of the peripheral zone isincreased slightly relative to the central zone to avoid possiblewarping of the lens over time by lid pressure applied thereto. It ispossible to change the thicknesses of the zones simply, allowing greatflexibility in designing the central and peripheral zones in a desiredmanner to achieve proper lens stability as well as to allow properoxygen transmission and tear flow beneath the lens. To change the centerthickness, a designer may vary the delta r value for the junctionbetween the base curve and the sigmoidal curve as shown at 222. Thiscontrols the thickness of the lens at that location, and allowscalculation of the true center thickness at 224 based upon the power ofthe lens. Again, it is possible to limit the variables to simplify thedesign of a lens which would cover most patient conditions, butflexibility exists in the lens design and system according to theinvention, to facilitate achieving the desired objectives for a givenpatient.

Example 2

For patient 2, having a prescription as follows:

42.00×44.00@165, Rx−3.50−1.00×160, e=0.6, HVID 11.4

With patient 2, a relatively high refractive error along with highastigmatism is noted, which again will lead the lens designer toincrease the value of the ellipticity of the cornea as shown at 230 inFIG. 16. Due to the high astigmatism, it is desired to have a thickercenter in the lens, which again is easily accommodated by varying thedelta r for the first junction between the base curve and the sigmoidalcurve as shown at 232. Other aspects of the particular lens design forpatient 2 are determined in a manner similar to that described withreference to patient 1 in Example 1. Particular values for thevariables, including base curve, lens diameter, angle of the peripheralzone as well as depth of the sigmoidal curve are shown in FIG. 16, withthe relationship between the zones shown graphically in FIG. 17. Therelatively thick center and thinner edge portion in this lens design isvisually represented in FIG. 19, with individual and cumulative volumesunder the lens shown in FIG. 18. FIG. 20 shows the relationship of thefront and back surfaces in each zone relative to one another.

Example 3

For patient 3 having a prescription as follows:

46.50×46.50@180, Rx−6.00−0.75×90, e=0.4, HVID 11.2

With reference to FIGS. 20-24, a lens design according to the presentinvention for patient 3 is shown. Selecting parameters of the basecurve, base diameter, angle of peripheral zone as well as depth of thesigmoidal curve are selected in a manner similar to that previouslydescribed, providing a first indication of fit to the lens designer. Forpatient 3 using a value of 3.0 mm as the radial distance from the lenscenter to the first junction, which may in general be held uniform,produced a relatively large void space adjacent the base curve at thefirst junction. The lens designer therefore has the flexibility toreduce or narrow the optical zone, as indicated by the use of 2.5 mm at210 and increases the width of the sigmoidal curve indicated at 212 as2.0. The relationship of the zones relative to one another and thecornea are shown in FIGS. 21-24, wherein the void space adjacent thebase curve at junction 1 was reduced.

Example 4

For patient 4 having a prescription as follows:

41.50×43.00@180, Rx−5.00−0.75×180, e=0.3, HVID 11.9

Patient 4 has a large refractive error, as well as a large HVID,translating to a relatively larger lens diameter than previous examplesas shown at 209. At the same time, patient 4 has a low ellipticity, suchthat proper support for the lens may not be provided at peripheralregions. In this circumstance, it is possible that lens might warp orbend out on the outside peripheral edges due to the low ellipticity,such that the designer may wish to thicken the outside portion of thelens. At the same time, thickening the peripheral regions of the lenswill reduce oxygen transmission to the cornea, and therefore thedesigner may wish to reduce the thickness of the central zone to allowbetter oxygen transmission. As shown in FIGS. 24-28, a lens designedaccording to the invention for patient 4 may accommodate such changes byreducing the delta r translation of the first junction at 222 andincreasing the delta r translation of the second junction at 242.Although these modifications allow redistribution of mass toward theperipheral region of the lens as shown in FIG. 27, and reduces thethickness of the central zone, the lens designer may also want to extendthe edge profile and control the edge shape to minimize discomfort. Inthis example, the edge lift provided in a desired design is startedearlier to reduce the mass at the very peripheral regions of the lens asshown in FIG. 29 reducing the mass at the edge where the eyelid willengage the lens facilitates comfort. To facilitate lifting the edge inthis manner, the parameters of the ellipse creating the posterior curveedge are modified at 243, 244 and 245 to facilitate imparting thedesired edge lift and promoting tear flow and oxygen transmission.

Example 5

For a patient 5 having a prescription as follows:

43.50×44.00@180, Rx−3.00, e=0.7, HVID 11.0

For patient 5, it is noted the cornea is very spherical with noastigmatism. In such a case, lens stability concerns are minimized, andthe lens may be made thin at central and peripheral portions. Reducingthe thickness of the lens may be accomplished by reducing the delta rtranslation points for the first and second junctions as shown at 222and 242 in FIG. 30. FIGS. 31-34 show the lens design for patient 5 inmore detail.

In the above examples, the prescriptions represent a very wide range ofmyopic corneas. It should also be recognized that the lens design canaccommodate patients having hyperopia. In such a lens, the final desiredshape of the cornea is achieved by redistributing corneal tissue to forma steeper corneal surface. Thus, such a design would typically use asteeper base curve accordingly, which in turn would suggest a greaterapical separation between the cornea and lens to ensure the base curvedoes not penetrate the cornea when analyzed on a model cornea. Thecentral zone may also be narrower, which again is easily accomplished bywidening the connecting zone in the lens design. The peripheral zone mayalso need not be elevated from the cornea at initial stages to thedegree a myopic design would, due to the correction to be imparted tothe corneal shape, as the lens will effectively be squeezing the corneafrom a larger annular zone to fill a smaller central zone of the lens.

Example 6

For a patient 6 having a hyperopic prescription, the lens was designedas follows: The lens design for the hyperopic condition of patient 6 isshown in FIGS. 35-39. The lens is designed to provide a lens/corneapower difference for patient 6 of 2.0 diopters as indicated at 200 inFIG. 35. The selected base curve has a 7.50 mm radius, as indicated at208. The power difference may again be selected as slightly more thanthe degree of correction required for the patient, if desired. Otherparameters of the lens may be chosen similarly to that previouslydescribed, such as lens diameter. The lens is then designed to have anarrower central zone, with the radial distance from the lens center tothe junction between the base curve and the sigmoidal curve, indicatedat 210, being reduced to 2.5 mm in this example. In turn, the width ofthe sigmoidal curve at 212 is increased to 1.5 ram. The height above thepretouch peripheral zone and cornea is somewhat less in this example,indicated as 0.024 mm at 244. Similarly, the volume between the pretouchperipheral zone and cornea is less in this example, indicated as 0.491(uL) at 246, and as shown in FIG. 37. The apical separation of the lensfrom the cornea is chosen to ensure the base curve doesn't penetrate thecornea, as indicated in the graph of FIG. 36 at 250.

As previously mentioned, it may be desirable in practice to limit thenumber of variables which are modifiable to design the lens forsimplifying the design process. As an example, lenses can be designedlimiting the variables to BC, DIA, sigmoidal curve depth and peripheralzone angle. In the examples, additional parameters were then modified asneeded to alleviate any problems in properly fitting the unusualpatient. All patients were successfully fit as the graphs and measuresshow. Since the lenses and patients in the examples represent relativeextremes, one could imagine unusual problems that might occasionallyarise with such patients. Other variables beyond the BC, DIA, sigmoidalcurve depth and peripheral zone angle can then be used to treat thesespecial cases. Additional variables include but are not limited to:Thick center/thin edge, such as shown in Example 2, which has very highcylinder and eccentricity warpage was possible. For Patient 3 which hada very high correction, requiring a big discrepancy between cornealradius and base curve, this led to a large volume at junction 1 so inthis case the central zone diameter was reduced and the connector zonewidened with the expected result bringing the connector closer to thecornea and reducing bubble possibilities. The low eccentricity exhibitedby patient 3 has led to high edge lift and junction 2 elevation. Asmaller diameter and lower angle would solve this problem if the patientfound the lens uncomfortable.

Patient 4 was prescribed a very large diameter lens, allowed by theirlarge HVID to assure good centration on his high correction and cylinderThis will reduce oxygen under the lens from tear movement, and a thincenter was provided. Extra thickness on the outside was added tominimize warpage but to make this thick edge comfortable the edge zonewas extended to 2 mm and the dividing line moved away from the basecurve for extra tear pumping.

Patient 5 is very spherical with high eccentricity, such that the lenswill hug the cornea, thickness could lead to excessive movement so thispatient offers an opportunity for extra oxygen and comfort with the thinprofile throughout. As seen in FIG. 18 as an example, showing therelationship between front and back surfaces, the anterior surface isdesigned to substantially mirror the posterior surface zone 12 andperipheral zone 26, while the connecting zone 14 is designed such thatthe posterior and anterior curves deviate from one another to apredetermined degree. As mentioned previously, the front or anteriorsurface may be designed such that there is different spacing between thesurfaces from the apical point to the peripheral edge of the lens, forexample to provide better oxygen transmission to the center of the lensand yet provide desired structural support at the periphery of the lens.The spacing difference is easily adjusted by adjusting the length andwidth of the anterior sigmoidal curve as compared to the posteriorsigmoidal curve. As shown in FIG. 17, a semi-meridian section of analternative embodiment is shown to include varying thickness toward theperiphery of the lens.

In accordance with the lens design as described according to the presentinvention, it is then possible to provide a method of fitting a patientwith an CRT for treatment, such as shown in FIG. 40. The patient'svisual acuity and corneal curvature is measured at 350 to determine thepresent shape of the cornea and enable a practitioner to select a basecurve for correction of the corneal shape to a desired degree. Thedetermination of the base curve of the central zone to affect desiredcorneal reshaping at 352 is then made, providing the design of thecentral zone 12. Thereafter, at 354, the diameter of the lens isdetermined, such as by measurement of the HVID. The slope of theperipheral zone may then be selected at 356, to in effect provide thedesign of the central zone 12 and peripheral zone 26 relative to a givenpatients cornea and the desired correction to be imparted. With thisinformation, the sigmoidal curve of the connecting zone 14 is designedat 358 for connecting the base curve 12 and peripheral zone 26. Theproper rectangle depth for the sigmoidal curve which leaves the tangentpoint elevated to a predetermined degree above the cornea can bedetermined at 358, with the fitter able to compare the lens design to amodel eye, by fluoroscene strips with trial lenses, or topographicalinformation. If necessary, the relationship of the lens zones to thecornea can be adjusted by varying the connector zone depths. The lensdesign can then be compared to a model eye using software, and triallenses can be fit on the patient to verify the lens design, andparticularly the design of each of the independent zones 12, 14 or 26 asshown at 360.

As previously mentioned, the method of fitting as described may accountfor a series of lenses, designed to progressively impart partialcorrection to the corneal shape until the final desired correction isachieved. In the method of fitting, adjustment of the lens design,visualization and assessment of the lens design, and the ability toteach and communicate design variables to a fitter are facilitated andenabled by the lens design itself. The method of fitting could utilizeadjustments in the sagittal depth of the lens 10 to adjust the cornealreshaping characteristics of the lens by changing the axial length ofthe connecting zone 14. Adjustments of the axial length of theconnecting zone 14 result in directly corresponding changes in thesagittal depth of the lens 10. Similarly, the method of fitting mayinclude varying the volume distribution of the void space createdadjacent the base curve 12 in association with the connecting zone 14.As previously mentioned, the characteristics of this void space enablecorneal tissue to be redistributed in the desired manner, and to allowflow of tears beneath the lens. Changes to the volume of this space maybe provided by varying the diameter of the central zone 12, the axiallength of the connecting zone and the radial width of the connectingzone, without otherwise affecting the lens design and fit. The method offitting also allows changes to the radial location of possibletangential contact of the redistributed cornea to the peripheral zone 26by varying the angle of the peripheral zone 26 to the central axis ofthe lens, again without otherwise affecting the lens design and fit. Theedge lift which may be desired in the peripheral zone 26 is also easilyadjusted by changing the extension of the lens beyond the point ofpossible tangential contact of the peripheral zone with the cornea ofthe wearer. The edge profile itself is also easily modified by changingthe axes of imaginary ellipses and the location of the imaginarydividing line between the posterior and anterior ellipses.

The method of fitting can thus allow the manufacture of a lens sethaving the central zone diameter, connecting zone width, lens diameterand edge profile provided with predetermined shapes. With such a lensset, the fitter then measures the preferred corneal curvature needed toeliminate refractive error for a patient, and may measure the centralcorneal curvature of the patient's cornea. Thereafter, the fitter needonly determine two parameters, the connecting zone depth and peripheralzone angle from fitting or computer modeling. As previously described,the parameters of connecting zone depth and peripheral zone angle may bederived by fitting lenses from a fitting set having a fixed connectingzone depth with a series of base curves or a fixed base curve with aseries of connecting zone depths and another set having a fixedconnecting zone depth with a series of peripheral zone angles from whichthe final selection is derived. Again, it is also possible to providethe set of fitting lenses with a plurality of visible concentric ringsas mentioned with respect to FIG. 4, to determine the lens diameter atwhich substantially tangential touch occurs between the lens and thecornea, and thereby determine the angle of the at least one peripheralzone.

As stated previously, the fitting of the lenses is simplified as thefitter is able to easily visualize the fit of the lens in associationwith a patient cornea. In FIG. 41, a schematic representation of thepatients eye and cornea are shown with the lens design positionedthereon. The actual corneal surface is represented by the ellipse at452. In assessing the fit of the lens 450 on the cornea 452,particularly at the peripheral regions where the peripheral zone of thelens is positioned, the cornea can be approximated by a circle 454 overa portion of the corneal surface. As can be seen, the circle 454 istangent to the ellipse 452 at the peripheral portion of the cornea. Todetermine the appropriate fit of the lens 450, the characteristics ofthe lens can be determined and changes in parameters visualized. Frommeasurement or observation, the fitter provides information regarding atest lens 450 positioned on the patient's cornea, particularly the angle(A1) used in the lens of the test series. As seen in FIG. 42, the lens450 tangentially touches the cornea represented by the circle 454 at 456relating to the touch diameter 1 (TD1). The optimum location, or touchdiameter 2 (TD2), for the tangential touch of the peripheral zone as setforth previously is then determined, and shown at 458, yielding a changein the height and width of triangles formed between the center of circle454, the touch diameters TD1 and TD2 and the line perpendicular to avertical radius of the sphere 454. With these parameters identified, thechange in the height and width result in a change in the connecting zonedepth (RZD). The radius of the sphere 454 to be determined relating tothe spherical cornea which would have a tangential contact with a lineat angle A1 from a line perpendicular to a vertical radius of the sphereby Eq. 9:

$\begin{matrix}{R = \frac{{TD}\;{1/2}}{{SIN}\;\left( {A\; 1} \right)}} & \left( {{Eq}.\mspace{14mu} 9} \right)\end{matrix}$where H is the distance along the vertical radius of the sphere from theorigin to the intersection with the sag diameter passing through thetangential point when A1 is fitted by Eq. 10.

$\begin{matrix}{{H\; 1} = \frac{{TD}\;{1/2}}{{TAN}\;\left( {A\; 1} \right)}} & \left( {{Eq}.\mspace{14mu} 10} \right)\end{matrix}$Using R from above, we can calculate TD2 and H2 by Eqs. 11 and 12:TD2=2*R*SIN(A2)  (Eq. 11)H2=R*COS(A2)  (Eq. 12)The difference between the height for A1 and A2 is found by subtraction,as is the difference in sagittal diameter. Both of these values arearranged to yield positive values when A2>A1 by Eqs. 13 and 14.ΔH=H1−H2  (Eq. 13)ΔW=2*(TD2/2−TD1/2)  (Eq. 14)Considering the smaller triangles in the upper right of FIG. 42, where xand y are the horizontal and vertical components respectively, thesecoordinates are found as follows:x1=TD1/2−J2  (Eq. 15)y1=x1*TAN(A2)  (Eq. 16)x2=TD2/2−J2  (Eq. 17y2=x2*TAN(A2)  (Eq. 18)

From this, the change in the connecting zone depth RZD is determinedΔRZD=y1−y2  (Eq. 19)with the relationship between peripheral zone angle and connecting zonedepth seen to correspond in a manner the fitter can visualize and verifyproper fit of the lens. In the example shown in FIG. 42, the RZD changeis only the component due to an angle change, and base curve changesmust be independently considered. In this example, the originalparameters observed relating to the fit of the fitting lens and theresultant changed parameters are given as follows:

J2 radius 4 Angle A1 −32 RZD change due to angle change only Angle A2−33 −0.023 Touch Diameter 9.75 10.02083 touch diameter after change

The present invention also provides for a method of establishingcentration over the visual axis of the lens by adjusting the location ofpossible peripheral tangential contact and the extension of the lensbeyond the point of peripheral contact of the lens with the cornea. Thisability in the lens design would allow better fitting of any contactlens, not just for orthokeratological treatment, and would enhancecomfort and provide other advantages by maintaining centration of thevisual axis over the cornea.

The present invention is also directed at a computer program product fordesigning orthokeratology contact lenses. A person of ordinary skill inthe art would appreciate that the invention may be embodied as a method,data processing system, or computer program product. As such, thepresent invention may take the form of an embodiment comprised entirelyof hardware, an embodiment comprised entirely of software, or anembodiment combining software and hardware aspects. In addition, thepresent invention may take the form of a computer program product on acomputer-readable storage medium having computer-readable program codeembodied in the medium. Any suitable computer-readable medium may beutilized including hard disks, flash memory cards, CD-ROMs, opticalstorage devices, magnetic storage devices or the like.

The method of fitting and the computer program product of the inventionare described with reference to flow charts or diagrams that illustratemethods, and systems, and the computer program product. It should beunderstood that each block of the various flow charts, and combinationof blocks in the flow charts, can be implemented by computer programinstructions. Such computer program instructions can be loaded onto ageneral-purpose computer, special purpose computer, or otherprogrammable data processing device to produce a machine, such that theinstructions that it executes on the computer or other programmable dataprocessing apparatus, create means for implementing the functionsspecified in the flow charts. The computer program instructions can alsobe stored in a computer-readable memory that directs a computer or otherprogrammable data processing device to function in a particular manner,such that the instructions stored in the computer-readable memoryproduce an article of manufacture including instruction means whichimplement the functions specified in the flow charts or diagrams. Thecomputer program instructions may also be loaded onto a computer orother data processing apparatus to cause a series of operational stepsto be performed on the computer, to produce a computer implementedprocess, such that the instructions which execute on the computer orother programmable apparatus provide steps for implementing thefunctions specified in the flow charts or diagrams.

It will also be understood that blocks of the flow charts supportcombinations of means for performing the specified functions,combinations of steps for performing the specified functions and programinstructions means for performing the specified functions. It is also tobe understood that each block of the flow charts or diagrams, andcombination of blocks in the flow charts or diagrams, can be implementedby special purpose hardware-base computer systems which perform thespecified functions or steps, or combinations of special purposehardware and computer instructions.

The software program of the present invention could be written in anumber of computer languages, and any suitable programming language iscontemplated. It is also to be understood that various computers and/orprocessors may be used to carry out the present invention, includingpersonal computers, main frame computers and mini-computers.

In FIG. 42, the user first inputs the corneal apical radius andellipticity, as well as the suggested base curve for correction ofvisual defects of a patient at 400. It is then determined whether a basecurve is available for total correction of the visual defects of thepatient at 402. If not, the user selects a base curve for partialcorrection at 404, or if such a base curve is available, the user inputsthe base curve and the diameter of the selected curve at 406. The userthen inputs the parameters of the diameter at 408, and selects theparameters of the peripheral zone angle at 410. The parameters of thesigmoidal curve are input at 412 as well as the lens power at 414. Theparameters of the edge design may be input at 416. The lens is designedusing the input parameters, and the relationship of the designed lens toa model cornea is determined at 418. From this analysis, it isdetermined whether the lens design is acceptable at 420, and if not thesystem returns to selection of a base curve at 402 or redesign of theother zones for redesign of the lens.

In an embodiment of the invention, a hand-held computer, such as aPersonal Digital Assistant (PDA), is programmed with the computerprogram of the invention, to compute the best lens fit from the fittersobservations. The program may utilize different approaches as previouslydescribed, such as the angle series of fitting lenses wherein thefitting lens set has a fixed base curve with a series of connecting zonedepths and another set having a fixed connecting zone depth with aseries of peripheral zone angles from which the final selection isderived. Another approach as described may utilize the fitting lenseshaving concentric visible rings to determine the diameter of the desiredtangential touch and compute the angle of the peripheral zone. In bothmethods, the computer program will prompt the user for inputs relatingto a flat keratometry reading on the patient, the patients refractiveerror, the final target refractive error (usually plano but may bedifferent), the horizontal visible iris diameter and the lens code forthe fitting set lens that just touched centrally and peripherally. Theprogram will prompt the user for the angle of the lens from the angleseries of fitting lenses whose tangential touch was at the preferredlocation as described. The other method requests the diameter at whichfitting set lenses (all having the same angle) displayed their ring oftangential touch.

Upon determining an acceptable design, lathe parameters and cutting datais calculated and generated at 422. As previously mentioned, based uponthe lens characteristics according to the invention, it is possible toprovide “unfinished base curve buttons” to be inventoried by lensfinishing labs or other similar entities. Using the unfinished basecurve button, a lens finishing lab may be given the lathe parameters andcutting data for a particular lens, which are simply downloaded to acomputer controlled lathe for generating the particular lens design fora patient. In the unfinished base curve button, the buttons may beprovided with the maximum diameter to be commercially provided, suchthat when the fitter specifies the required diameter, the lenses can becut down to that diameter in the area of the peripheral zone without anyeffect upon the rest of the precut portions of the lens. Thus, onebutton having particular base curve and sigmoidal curve characteristicsmay be used for all possible peripheral zone diameters. Further sinceall aspects of lens optical power are provided on the anterior surfaceof a finished lens, a base curve selected to fit a particular patientmay be employed to make a lens of nearly any optical power. Thisinventory advantage exists even if the button is already provided with apredetermined diameter and edge contour. In this way, the number ofbuttons to be inventoried is minimized, while providing significantflexibility in the ultimate lens design. The ability to provide lathecutting instructions to the finishing lab also greatly simplifiesmanufacture of a lens according to the invention, again greatlyfacilitating use of such lenses as well as reducing costs thereof.

The present invention may also accommodate multifocals and astigamaticlenses, both of which use the design according to the invention, buttoric lenses for astigmatism and/or improved peripheral fit (onnon-spherical eyes). With the design approach, this allows the designerto choose two orthogonal meridians of corneal shape, and designing acorresponding portion in the lens for each separately. Present lathetechnology can accept designs that vary in two meridians, and the designof the invention makes programming these lathes as easy as non-toroidaldesigns. One simply subtracts the z axis value of one meridian at each xpoint and uses this data as difference data tote used by the latheduring each rotation. The technique is not limited to two orthogonalmeridians, but could incorporate many such meridians.

One of the benefits of the present invention is the ability to preciselycontrol the elevation of the lens center with respect to the cornealsurface. In Ortho-k, the center of the lens may contact the centralcornea, but some situations exist where one wishes to minimize oreliminate this contact. In normal rigid lenses this control is obviatedby the use of base curves that mimic the corneal surface, and thus nochange arises as a result of the contact. But in some situations, it isdesirable to have a different geometry on the base curve. Presbyopiclenses with multiple curves are a case in point though other situationsalso exist. Being able to support a lens off of the corneal surface canmake many new base curve geometry designs possible, as multifocals arean example. It is also possible to provide the base curve with ageometry which when that geometry is impressed on the cornea could makeit multifocal. Such base curves may thus be formed with one or morespherical or aspherical zones to provide these characteristics. Thebenefits are obvious, such as allowing a patient to wear contact lenseswhile they sleep to avoid wearing reading glasses during the day.Examples of other applications for the present invention could alsoinclude providing a corneal shape controlling device to control orimprove laser surgery. Use of the invention could reduce failures andannoying compromises.

The foregoing disclosure is illustrative of the present invention and isnot to be construed as limiting the invention. Although one or moreembodiments of the invention have been described, persons of ordinaryskill in the art will readily appreciate that numerous modificationscould be made without departing from the scope and spirit of thedisclosed invention. As such, it should be understood that all suchmodifications are intended to be included within the scope of thisinvention. The written description and drawings illustrate the presentinvention, and are not to be construed as limited to the specificembodiments disclosed.

What is claimed is:
 1. A computer program product for designing acontact lens comprising: a computer usable storage medium, a computerreadable program code, responsive to user inputs, for modeling a contactlens to have a central zone having a posterior surface curvatureselected according to characteristics of a patient's cornea, and a firstand at least one second annular zone wherein the at least one secondannular zone is positioned and shaped to engage said cornea uponalteration of its shape a predetermined amount, and the first annularzone formed as a sigmoidal curve connecting the central zone and the atleast one second annular zone, and computer readable program code forcalculating cutting parameters for a lathe used to produce said lensfrom a blank of material.
 2. The computer program product according toclaim 1, wherein the first annular zone is defined by at least aparameter of zone depth and the at least one second annular zone angleis defined by at least a parameter of it's angle relative to the cornea,wherein these parameters are derived by calculating a best fit lens frommeasurements comprising a flat keratometry reading of the patient'scornea, the patient's refractive error, a final target refractive error,a horizontal visible iris diameter, and a lens code relating to a lensfrom a fitting set of lenses which contacts the cornea of the patient ina manner to cause redistribution of corneal tissue when placed on thecornea of the patient.
 3. The computer program product according toclaim 2, wherein the fitting set of lenses are selected from the groupof fitting lenses having a variable series of base curves with a fixedconnecting zone depth and a fixed peripheral zone angle, a fixed basecurve with a variable series of connecting zone depths and a fixedperipheral zone angle, variable series of base curves with a fixedconnecting zone depth and a plurality of concentric rings, a fixed basecurve with a variable series of connecting zone depths and a pluralityof concentric rings or having a fixed base curve and a fixed connectingzone depth and a series of peripheral zone angles with or without aplurality of visible concentric rings.
 4. The computer program productaccording to claim 2, wherein the lens selected from the fitting lenseshas a second annular zone angle which tangentially touches the cornea ata desired location, and the diameter of the lens at which the tangentialtouch occurs is input to model said contact lens.
 5. The computerprogram product according to claim 4, wherein the fitting lenses have aplurality of visible concentric rings provided thereon to allow thediameter of the tangential touch to be determined.
 6. The computerprogram product according to claim 5, wherein the angle of the secondannular zone is calculated from the determination of diameter oftangential touch observed when fitting another fitting lens havinganother angle for the second annular zone.
 7. The computer programproduct according to claim 1, wherein the program accepts inputsrelating to the corneal apical radius and ellipticity of a patient'scornea, as well as at least a first base curve associated with thecentral zone for correction of visual defects of a patient byredistribution of corneal tissue when placed on the cornea of thepatient, the program further accepting inputs relating to the diameterof the lens, the second annular zone angle, the parameters of thesigmoidal curve of the first annular zone and the lens power, whereinthe program creates a lens design, and displays the relationship of thedesigned lens in comparison to a model cornea to determine whether thelens design is acceptable for use on the patients cornea.
 8. Thecomputer program product according to claim 1, wherein the a computerreadable program code is provided in a hand-held computer.
 9. Thecomputer program product according to claim 1, wherein the programutilizes user inputs relating to a fixed base curve with a series ofconnecting zone depths and a fixed first annular zone depth with aseries of second annular zone angles.
 10. The computer program productaccording to claim 1, wherein the program utilizes user inputs relatingto observations using a set of fitting lenses having concentric visiblerings to determine the diameter of the desired tangential touch of thesecond annular zone and compute the angle of the second annular zone.11. The computer program product according to claim 1, wherein theprogram prompts a user for inputs relating to a flat keratometry readingon the patient, the patients refractive error, the final targetrefractive error, the horizontal visible iris diameter and the lensparameters for a fitting set lens that just touched centrally andperipherally the patients corneas, and the angle of the second annularzone.
 12. The computer program product according to claim 1, wherein theprogram prompts a user for inputs relating to the diameter at which aset of fitting lenses having concentric visible rings to determine thecomputed diameter of the desired tangential touch of the second annularzone.
 13. The computer program product according to claim 1, wherein theprogram allows a lens designer to choose a plurality of orthogonalmeridians of corneal shape, and design a corresponding portion in thelens for each separately.
 14. The computer program product according toclaim 1, wherein the program allows a lens designer to control theelevation of the lens center with respect to the corneal surface.